Standards for wireless local area network (WLAN) technology have been and continue to be developed by the institute of electrical and electronics engineers (IEEE) as the 802.11 standard. IEEE 802.11a and b use an unlicensed band at 2.4 GHz or 5 GHz, IEEE 802.11b provides a transmission speed of 11 Mbps, and IEEE 802.11a provides a transmission speed of 54 Mbps. IEEE 802.11g applies orthogonal frequency division multiplexing (OFDM) at 2.4 GHz to provide a transmission speed of 54 Mbps. IEEE 802.11n applies multiple input multiple output-OFDM (MIMO-OFDM) to provide a transmission speed of 300 Mbps for four spatial streams. IEEE 802.11n supports a channel bandwidth up to 40 MHz and in this case, provides a transmission speed of 600 Mbps. IEEE 802.11af is a standard defining an operation of an unlicensed device in a TV whitespace (TVWS) band.
TVWS is a frequency band allocated to a broadcast TV, which includes an ultra high frequency (UHF) band and a very high frequency (VHF) band and in which an unlicensed device is allowed to operate so long as such operation does not interfere with that of a licensed device. Examples of a licensed device may include a TV, a wireless microphone, and so on. The licensed device may be called an incumbent user or a primary user. In addition, in order to overcome problems in terms of coexistence between unlicensed devices using the TVWS, a frequency sensing mechanism, a signaling protocol such as a common beacon frame, and so on may be needed.
According to TVWS, operations of all unlicensed devices, except for in certain special cases, are allowed at 512 to 608 MHz and 614 to 698 MHz. However, only communication between fixed devices is allowed in a band of 54 to 60 MHz, 76 to 88 MHz, and 174 to 216 MHz, and 470 to 512 MHz. The fixed device refers to a device for transmitting a signal at a predetermined location only. An IEEE 802.11 TVWS UE refers to an unlicensed device that operates using an IEEE 802.11 media access control (MAC) and a physical layer (PHY) in a TVWS spectrum.
An unlicensed device that desires to use TVWS needs to provide a function of protecting a licensed device. Thus, the unlicensed device should check whether the licensed device occupies a corresponding band prior to transmission of signals in the TVWS.
To this end, the unlicensed device may perform spectrum sensing to check whether the corresponding band is being used by the licensed device. Examples of spectrum sensing mechanism may include an energy detection method, a feature detection method, and so on. When a signal received at a specific channel is greater than or equal to a predetermined value or a DTV preamble is detected, the unlicensed device may determine that the licensed device is using the specific channel. In addition, when the unlicensed device determines that the licensed device is being used in a channel adjacent to a currently used channel, the unlicensed device needs to reduce transmit power thereof.
In addition, the unlicensed device needs to access a DB through the Internet or a leased line to acquire channel list information that is available by the unlicensed device in a corresponding area. The DB stores and manages information regarding licensed devices registered therein and channel usage information that dynamically varies according to geo-location and usage time of corresponding licensed devices.
Through this specification, a whitespace band includes the aforementioned TVWS but is not limited thereto. The term whitespace band as used herein refers to a band in which an operation of a licensed device is preferentially allowed and an operation of an unlicensed device is allowed only when the licensed device is protected. In addition, a whitespace device operates in a whitespace band. For example, a device based on an IEEE 802.11 system is also an example of the whitespace device. In this case, the whitespace device may refer to an unlicensed device that operates using an IEEE 802.11 MAC layer and PHY layer in the whitespace band. That is, a general 802.11 standard-based AP and/or STA operating in the whitespace band may be an example of the unlicensed device.